Lysosomal storage diseases result from an inherited trait which affects the levels of enzymes in the lysosome. Approximately 30 lysosomal storage diseases are known to affect humans. Tay-Sach's disease and Gaucher disease are members of this group of diseases. Since specific pathways for the uptake of these other lysosomal enzymes also exist, enzyme replacement therapy is expected to be effective in Fabry disease and could logically be expected to be successful in these other diseases as well. Although these diseases are individually rare, (e.g., several thousand patients with Fabry disease are known to occur world wide, i.e., 1 to 40, 000), as a group this class of diseases accounts for a significant fraction of all inherited diseases.
Fabry disease is one such lysosomal storage disease known to affect humans. Fabry disease is an X-linked inborn error of metabolism resulting from a deficiency of the lysosomal enzyme, α-galactosidase A. Deficiency of α-galactosidase A results in the accumulation of its major glycosphingolipid substrate, globotriaosylceramide and related glycolipids with terminal α-galactosidic linkages. Progressive substrate deposition, especially in the plasma and vascular endothelium, leads to ischemia and infarction with early demise due to vascular disease of the heart, kidney, and brain.
Enzyme replacement therapy was first suggested by De Duve et al. (See De Duve et al. 1964, Federation Proceedings 23:1045) as a possible approach to the treatment of lysosomal storage defects such as Fabry disease. Several lines of evidence suggest that enzyme replacement therapy may be beneficial for patients with Fabry disease. For example, it has been demonstrated in cell cultures of fibroblasts obtained from patients with this disease that enzyme present in the culture medium is specifically transported to lysosomes. Clinical trials of enzyme replacement therapy have been reported for patients with Fabry disease using infusions of normal plasma (Mapes et al., 1970, Science 169: 987-989); α-galactosidase A purified from placenta (Brady et al., 1973 New Eng. J. Med. 279: 1163); or α-galactosidase A purified from spleen or a (Desnick et al., 1979, Proc. Natl. Acad. Sci. USA 76: 5326-5330). In one study (Desnick et al.) intravenous injection of purified enzyme resulted. in a transient reduction in the plasma levels of the substrate, globtriaosylceramide.
Since the initial studies of enzyme replacement therapy as a potential treatment for Fabry disease, Calhoun et al. have isolated human α-galactosidase A cDNA (Calhoun et al. 1985, Proc Natl Acad Sci USA 82:7364-8) and genomic clones (Quinn et al. 1987, Gene 58:177-88), and expressed the cDNAs in bacteria (Hantzopoulos et al. 1987, Gene 57:159-69), insect cells (Chen et al. 2000, Protein Expr Purif 20:228-36; Coppola et al. 1994, Gene 144:197-203), and Pichia pasioris (Chen et al. 2000, Protein Expr Purif 20:472-84). Furthermore, the purified recombinant α-galactosidase A produced in insect cells and P. pastoris has been shown to be taken up by Fabry fibroblasts in cell culture.
Several clinical trials of enzyme replacement therapy for Fabry disease patients in the last few years revealed clinical efficacy. Furthermore, the FDA has approved treatment of Fabry disease through intravenous administration of recombinant α-galactosidase A. This treatment methodology is known as enzyme replacement therapy (ERT). ERT does not affect the underlying defect, but provides a functional enzyme for the cell. Currently, two galactosidase drugs are available for treatment of Fabry disease via enzyme replacement therapy (ERT): agalsidase alfa (Replagal®, TKT/Shire) and agalsidase beta (Fahrazymne®, Genzyme). These protein based therapeutics are administered by (approved for) intravenous injection and deliver galactosidase activity to the lysomomes of affected organs in order to reduce the level of globotriaosylceramide accumulation.
However, the current approaches for enzyme replacement therapy can be expected to have limitations 88% of patients developed potentially neutralizing IgG antibodies to α-galactosidase A (Fabrazyme®) with a therapeutic enzyme dose of 1 mg per kilogram of body weight (Eng et al. 2001, N. Engl. J. Med. 345: 9-16); while only 21% developed antibodies with a lower dose of 0.2 mg of enzyme (Replagal®) per kilogram of body weight, (Schiffmann et al. 2001, JAMA 285:2743-9).
Fabry disease patients with adverse reactions to the infusions are currently treated with antihistamines and antipyretics, but it can be anticipated that life-long treatment required for these patients will lead to unacceptable levels of neutralizing antibodies. Furthermore, the treatment regimen is burdensome. Infusions typically involve a 4-6 hours session, in a hospital setting, every other week for life. In a recent clinical trial 14 out of 58 patients had to withdraw due to serious noncompliance, voluntary withdrawal, serious adverse events leading to death due to heart or kidney disease due to symptoms from the disease, and IgE antibody or positive skin tests. See Germain D P et al. 2007, J. Am. Soc. Nephrol., 18(5):1547-57. Most patients suffer from ill-defined infusion-associated reactions (vomiting, rigors, etc.) and are routinely pretreated with acetaminophen and hydroxyzine, and some patients require ibuprofen, prednisone, or both, for infusion associated reactions. Any improvement in treatment that leads to lower doses of enzyme, more effective therapeutic effects, or less frequent infusions would markedly improve the lives of Fabry disease patients and other patients reeving enzyme replacement therapy for lysosomal storage diseases.
Hence there exists a long standing need to provide a treatment regimen that requires lower doses of enzyme, thereby providing for snore effective therapeutic effects and less frequent infusions. In particular, there is a need to provide an enzyme therapeutic that allow for targeted delivery within the body and are sufficiently biologically active upon intracellular uptake.